Indeed. Currently you can do that. In my case I simply use Facebook as a photo-sharing site. That's all the interest I have in it.

Sites that require a Facebook login (as some startups create) get instantly closed. Sites that offer Facebook login have that option ignored. Sites that nag you about using that feature (eg tv.com) make me irate.

But comments takes things a step further. What if you get widespread adoption of Facebook comments to the point that you can't comment on most things without logging on to Facebook?

At that point it's getting to the point where either Facebook is your online identity or you have no online identity.